of chat shell, but afterwards finding the young, and perceiving 

 that the umbo was not so far backward as in the present shell, 

 and the remarkable membrane, induced me to describe it as 

 new. Jt occurred on the rocky shore of the river, and appears 

 to be rare. 



U. purpuriatus. Transversely oval, slightly oblong, in some 

 specimens with a little tendency towards ovate; dirty yellow- 

 ish or fuscous, obscurely radiate with blackis.i-green: beaks, 

 behind the m'ddle, hardly raised: anterior edge, rounded, or 

 rather, composed of two nearly rectilinear lines: posterior mar- 

 gin rounded: hinge margin regularly rounded: cavity of the 

 hinge membranes (behind the beaks) narrow, but very obvious: 

 basal margin arquated, a little compressed in the middle, and 

 sometimes almost contracted in that part: within, purple; mar- 

 gin livid; anterior submargin iridescent: primary teeth nearly 

 direct, rather thick, striated; lateral teeth hardly extending be- 

 yond trie sinus of the hinge margin. 



Length, one inch and three-fifths; breadth, three inches 

 and one-tenth. Convexity, over nine-tenths of an inch. 

 Var. a. Within white: Var. b. Within dull yellowish. In- 

 habits Mexico. 



For this species, I am indebted to Mr. Maclure. It was 

 found by William Bennett in a stream a hw leagues from Vera 

 Cruz, where it is abundant. It resembles U. purpureas, Nob., 

 but differs in having the teeth more direct; in the lateral teeth 

 being shorter, with respect to the sinus of the hinge margin; 

 in having the greatest breadth at the middle of the anterior 

 vp t gin, k.c. 



In the young and middle aged specimens, the radii are very 

 distinct, but are obsolete in old specimens. The umbones are 

 widely decorticated in age, but seldom are they so deeply 

 eroded as to disclose the waxen colored stratum. 



It may be considered as the Mexican analogue of the pur- 

 pureas. 



17. m tallicus. Transversely ovate, very sightly oblong: dull 

 yellowish tinged with coppery, particularly on the umbones: 

 ■ ,ly elevated, not decorticated, undulated; the undula- 

 tions obsolete in their middles, and not extending on the um- 

 bo; placed far back: hinge margin prominently arquated: 

 posterior margin very short, rounded: anterior margin rounded: 

 basal margin subrectilinear: ruithin liver color: iridescent on 

 the anterior cardinal teeth rather long, somewhat tu- 



berculous, not very prominent: that of the right valve not 



